But was ecumenism a "heresy condemned by the holy councils or fathers"? You could argue that certain figures (St. Justin Popovich, etc.) condemned it, that certain Local groups condemned it (1983 anathema of ROCOR), that certain figures supported the old calendarists (St. Nikolai of Serbia), and so forth. But doesn't all this come long after the initial break which is supposed to be the schism? Even if you count Met. Chrysostomos of Florina as a "father" (which I think is a stretch, at best), he and the other bishops didn't break with the state Church until over a decade after the schism was supposed to have happened. I think you could argue that no holy fathers or councils had condemned ecumenism at the point that the old calendar movement started. Perhaps I would not go as far as the synod of Met. Cyprian and say that a council (pan-Orthodox or even ecumenical) was needed to condemn it before the "potential schismatics" because actual ones, but still, I think it's pushing it to argue for traditionalism using the 15th canon of the 1st-2nd.
I also don't think it does much good to appeal to this canon as it relates to the calendar, as a calendar change (especially regarding Pascha) was forbidden, but I'm not sure that you could persuasively argue that this rises to the level of heresy. Admittedly, if memory serves, there did seem to be some calendar issues in the early Church that seemed to be dealt with as a heresy (quartodecimans?), but I'm not sure that that's the case here*. The best route that I can see is demonstrating that the Fathers and/or Councils condemned the pan-heresy (or "heresy of heresies") of ecumenism, but I don't know what kind of case can be made for that (and I've read the various materials--translated/published in English, anyway). Perhaps I am protesting too much, or perhaps I am rambling; I just remain unconvinced.
*I'm leaving aside the sticky issue of Finland, and possibly other local groups and their changes.